If you are buying or remodeling a house in any of the historic neighborhoods of Cambridge or greater Boston, it can be useful to know about the history, and geohistory, of the building soils that underlay them. The urban landscape appears to be one undifferentiated expanse, but due to glaciation, there is some variety to the materials we find below grade. If you are noting any subsidence (sinking) or cracking in the foundation area, and are concerned about the stability of the structure, understand that this can be a function of the composition of the soils that it was built upon.
The glacier, as noted, has placed deposits of sand and gravel seemingly at random, according to its whim. This material furnishes stable bearing, if compacted somewhat, and in addition is well drained. The most reliable footing that we can hope to find is a stone ledge outcropping to which we can anchor our footing or foundation, but this is not usual. Such outcroppings are most common in Somerville, Brighton and parts of Brookline.
In north and west Cambridge, we find mostly clay, and indeed there were claypits and potteries there, historically. Clay furnishes stable and compact bearing, but poor drainage. Care must be taken to waterproof any below grade areas, or to provide sumps and drains.
Boston’s Back Bay was famously just that, a water feature, more specifically marshland. The soil itself can be described as organic silt, a material that is extremely unreliable as it is subject to decomposition. To support the dense masonry row housing that can be seen there now, masses of 30’ wooden pilings were driven into the muck, where they remain, more than 150 years later. As long as they remain below the water table, they will not rot and will remain capable of furnishing reliable bearing.
On the Cambridge side of the Charles River, south of Harvard Square and in Cambridgeport, a similar organic muck is found, including the remains of significant oyster beds. Here, the long departed developers simply dug an excavation, laid up their foundations and hoped for the best. This worked out fine for a century or so, until decomposition began to do its work, resulting in uneven settlement. Vibrations from vehicular traffic accelerated this subsidence, in some cases resulting in structural damage and even condemnation of the affected structures. There are measures that can be taken to provide support for these structures, short of driving piles. S + H establishes a new concrete footing upon helical piers, basically steel screws a foot in diameter driven as deeply into the muck as the engineer specifies, 20 feet, 30 feet or more.
Clearly, a building is only as stable as its foundation and that stability can depend greatly on its location. When you check out the neighborhood, look below the surface, because everything is built upon what you can’t see. A structural engineer or experienced general contractor will be able to tell you what you need to know.